Power of a Lens

The power of a lens (P) measures its ability to bend light rays. It determines how strongly a lens converges or diverges light. The degree of convergence or divergence depends on the focal length (f).

• Convex lens (converging lens): Focuses light rays closer to the principal focus.
• Concave lens (diverging lens): Spreads light rays away from the principal focus.

Formula for Power of a Lens: \[\mathrm{P}=\frac{1}{\mathrm{f}\left(\mathrm{m}\right)}\]

where f is the focal length in metres.

SI Unit of Power:

• Dioptre (D)
• Defined as: 1D = 1m⁻¹ (A lens with a 1m focal length has a power of 1 dioptre).

Nature of Power:

• Convex lens → Power is positive (P>0).
• Concave lens → Power is negative (P<0).

Effect of Focal Length on Power:

• Shorter focal length → Higher power (greater bending of light).
• Longer focal length → Lower power (less bending of light).
• Example: A convex lens with a short focal length converges light more strongly, while a concave lens with a short focal length diverges light more strongly.

Applications:

1. It is used in eyeglasses, microscopes, telescopes, cameras, and optical instruments.
2. Higher power lenses are used for stronger corrections in vision (myopia, hyperopia).